pi

2. (mathematics) The mathematical constant that is the circumference of a circle divided by its diameter. Pi is written as the greek character pi.

Some have suggested that pi is the wrong choice and a better constant to describe the geometry of circles would have been 2*pi, for which they have proposed the name tau. Most practising mathematicians think this is silly. The xkcd comic strip proposed a compromise between pi and tau, namely 1.5*pi or "pau".